twitter vocab.
if you understand the concept of hashtags, you'll understand "#hashtag."
it's basically a wild card for hashtags and leaves it open for interpretation for the reader.
it can even fill a void when no words can properly describe the extreme extent of the tweet (situation) perfectly. or use when you're having a hard time coming up with a clever hashtag.
when you can relate to the situation and/or picture it happening.

basically if you ever catch yourself saying "i dont know what to hashtag" use #hashtag